Host-switching does not circumvent the Ni-based defence of the Ni hyperaccumulator Streptanthus polygaloides (Brassicaceae)

Elevated tissue concentrations of metals have been shown to defend metal-hyperaccumulating plants against both herbivores and pathogens. Tolerance of metal-based defences presents a challenge to herbivores, because heavy metals cannot be degraded or metabolized. One strategy that herbivores can employ to counter high-metal defences is dietary dilution, or host switching. Highly mobile herbivores are most likely to use this strategy, but less mobile lepidopteran larvae can also Improve their performance on toxic hosts if early instar development occurs on more favourable hosts. We examined the effects of host switching on growth and survival of a generalist folivore. Specifically, we tested the hypothesis that early larval development on non-toxic hosts could improve larval performance of the beet armyworm, Spodoptera exigua, an high-Ni Streptanthus polygaloides, a Ni hyperaccumulator. Initial larval performance (weight gain) was lowest for insects switched to high-N! hosts. Decreased initial larval performance was also noted for insects switched from lettuce to low-Ni S. polygaloides, but these larvae recovered quickly. Original host identity (lettuce or low-Ni S. polygaloides) did not affect subsequent larval performance. By day 8 of the feeding trials, all larvae switched to high-Ni hosts had died. We conclude that polyphagous Spodoptera larvae are unable to counter NI-based defences via host switching.